This invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to Radio Frequency (RF) coils used in MRI systems.
To acquire images, MRI systems utilize nuclear spins of hydrogen in the water contained in a patient to be imaged. The nuclear spins of hydrogen are polarized by a magnetic field (B0), which is typically generated by a magnet. The patient to be imaged is placed in a bore of the magnet. The polarized nuclear spins generate magnetic moments in the patient that get aligned parallel to the direction of the B0. Data acquisition is performed by exciting the magnetic moments in a Region of Interest (ROI) in the patient with a uniform RF magnetic field (B1) orthogonal to B0. B1 is typically generated by an RF transmit coil. B1 excites the ROI and causes the magnetic moments to align away from B0 by a predetermined angle. As the magnetic moments precess around B0 axis, the magnetic moments release absorbed energy to return to a steady state. In releasing absorbed energy, the magnetic moments generate a magnetic resonance signal, i.e., the energy is released at a resonance frequency. This resonance signal is received by the same or different RF coil in a reception mode. The receive coil can, therefore, be either the transmit RF coil or a receive-only RF coil. The receive RF coil is typically positioned in the vicinity of the excited ROI.
Before scanning the patient to acquire an image, the MRI system may be calibrated by performing a prescan. A prescan is used to optimize the image acquisition, for example, for a specific body part and may include calculation of autoshim information or shim parameters, and Transmit Gain (TG) information. The TG information determines the RF power emitted by the transmitter. Calibration of the TG information is important because power absorption is dependent on patient size, and the TG information has to be adjusted for each patient. Further, inhomogeneities may be introduced in B0 due to the patient's tissue. The inhomogeneities can be compensated to some degree by small gradient magnetic fields called shim fields. Therefore, during calibration, shim parameters, such as current, are calculated and configured for compensation.
The RF coils used for scanning are also used for the prescan. The RF coils used may be volume coils or surface coils. Further the RF coils may be local or body coils. Local coils can transmit to or receive signals only from a small part of the patient, whereas the body coil generally has a larger coverage area and can be used to transmit or receive signals to the whole body of the patient. Using receive only local coils and transmit body coils provides a uniform RF excitation and good image uniformity. However, a high RF power is deposited in the patient. For a Transmit/Receive (T/R) local coil, the local coil provides the RF excitation to the ROI and receives the MRI signal. Use of T/R local coils, therefore, decreases the RF power deposited. Conventionally, the RF coils utilized in MRI systems are used in either quadrature mode or phased-array mode. RF coils in quadrature mode are sensitive to signals 90 degrees out of phase and are generally used when speed of reconstruction is critical. Alternately, the receiver coils may be operated in a phased-array mode in which the 90 degree offset signals are forwarded to separate receivers. Operation in phased-array mode is typically used when image quality is critical.
Generally, T/R phased-array RF coils are used in MRI systems. However, while using these coils only a single receive channel is used to calculate the TG information and shim parameters during the prescan. This may lead to less than acceptable calibration parameters, which may further lead to image quality degradation.